[iwyu] Take load average into consideration on CI
This required that we backport iwyu_tool.py from https://github.com/include-what-you-use/include-what-you-use/pull/891 into tools/, which supports --load.
Edited by Maxim Onciul
This required that we backport iwyu_tool.py from https://github.com/include-what-you-use/include-what-you-use/pull/891 into tools/, which supports --load.